1. Field of the Invention
The invention relates to a solid electrolyte of the aluminum oxide-and-sodium oxide type.
2. Description of the Prior Art
Such solid electrolytes are used, for instance in Na/S storage cells which are used for the construction of high-temperature storage batteries.
In rechargeable electrochemical storage cells of the sodium and sulfur type, the reactant spaces are separated from each other by a solid electrolyte which conducts sodium ions. The solid electrolytes used in the storage cells, which are, for instance, made of beta-aluminum oxide, are distinguished by the fact that the partial conductivity of the mobile ion is very high and the partial conductivity of the electrons is smaller by several powers of ten. By the use of such solid electrolytes for the construction of electrochemical storage cells there is practically no self-discharge, since the electron conductivity is negligible, nor can the reaction substances get through the solid electrolyte as neutral particles. An advantage of these storage cells is that no electrochemical secondary reactions occur during charging. The reason therefor again is that only one ion type can get through the solid electrolyte. The current yield of such a storage cell is therefore approximately 100%. Storage cells of the sodium and sulfur type are therefore being considered preferably for the design of high-temperature storage batteries.
The use of beta-aluminum oxides in the form of ceramic components in sodium/sulfur storage cells and in other electrochemical cells which are based on the practically exclusive passage of sodium ions or other alkali or earth alkali ions, of silver ions, of protons or proton-transporting compounds under the influence of a gradient of the electrochemical potential, is known from German Pat. No. 15 96 077. Typical sodium-ion-conducting beta-aluminum oxide consists primarily of 80 to 95% of aluminum oxide and 5 to 12% of sodium oxide; in oxide-ceramic technology it is customary to refer the components of the composition mass-wise to the oxides of the metals present.
From this composition it will be evident that beta-aluminum oxide cannot be a structural modification of the chemical compound aluminum oxide (Al.sub.2 O.sub.3), as is true, for instance with .alpha.--Al.sub.2 O.sub.3 or .gamma.--Al.sub.2 O.sub.3. The misleading name beta-aluminum oxide has historical reasons; sodium oxide (Na.sub.2 O) or chemically related compounds such as other alkali metal oxides, earth alkali metal oxides, silver oxide, water or other compounds containing water are essential components of this chemical compound.
There are at least four different beta-aluminum oxide structures, namely .beta.-, .beta."-, .beta."'- and .beta.""--Al.sub.2 O.sub.3 which have as a common feature the buildup of several successive densely-packed oxygen layers, which are separated block-wise by two-dimensional parallel layers with reduced oxygen content. These intermediate layers contain in addition to the oxygen ions, the mobile ions which are essential for the use of beta-aluminum oxides as solid electrolytes. Of the phases mentioned above, the .beta.- and the .beta."--Al.sub.2 O.sub.3 phases are preferred. .beta.--Al.sub.2 O.sub.3 has the theoretical composition A.sub.2 O11Al.sub.2 O.sub.3. The symmetry of the structure belongs to the space group P6.sub.3 /mmc. The cell unit has a repetitive crystallographic spacing along the c-axis of about 2.2 nm. Customarily associated with .beta."--Al.sub.2 O.sub.3 is the theoretical composition A.sub.2 O5.33Al.sub.2 O.sub.3. .beta."--Al.sub.2 O.sub.3 crystallizes in the space group R3m. The crystallographic spacing along the c-axis of the triple-primitive hexagonal elementary cell is approximately 3.4 nm. In the composition formulas, A stands for an alkali metal or silver or hydrogen and determines the ions that are mobile within the solid electrolyte. The formulas above can readily be changed for possible consideration of earth alkali metals or hydrogen-containing compounds as substitutes. Examples of hydrogen containing compounds may be a lower hydrocarbon, i.e. a hydrocarbon having 1 to 5 carbonations. The other symbols have the customary chemical meaning.
Of the two last-mentioned phases, the .beta.--Al.sub.2 O.sub.3 phase has the higher alkali ion conductivity. The .beta."--Al.sub.2 O.sub.3 phase which is customarily desired for this reason, and the customarily less desirable .beta.--Al.sub.2 O.sub.3 phase can be distinguished by conventional X-ray diffraction techniques (for instance with a Bragg-Brentano diffractometer) due to their symmetry. In the meantime it therefore has become state of the art to make beta-aluminum oxide ceramics, the by far largest part of which consists of the .beta."--Al.sub.2 O.sub.3 phase, which can again be determined by X-rays.
The addition of doping substances is known from German Pat. No. 15 96 078. These doping substances serve, not only for lowering the resistance of a ceramic component of beta-aluminum oxide to migration of the mobile cations under the influence of a gradient of the electrochemical potential, but also for the thermal stabilization of the .beta."--Al.sub.2 O.sub.3 structure during the firing of the ceramic.
The additions of lithium, magnesium or similar metals with a valence not exceeding 2 are always less than three mass % in solid electrolyte ceramics which are made in accordance with the known methods. The known magnesium-doped sodium ion-conducting beta-aluminum oxide ceramics always contain between 7 and 9% Na.sub.2 O and between 1.5 and 2.5% MgO, while the remainder consists of .beta."Al.sub.2 O.sub.3.
Recent investigations, using an X-ray diffraction technique with higher resolution (Guinier-camera), have shown that the known manufacturing methods, using the mentioned compositions, do not lead to a single-phase .beta."--Al.sub.2 O.sub.3 solid electrolyte ceramic. The so-manufactured ceramics (under some circumstances besides an also present .beta.--Al.sub.2 O.sub.3 phase inventory) have two partial .beta."--Al.sub.2 O.sub.3 phases. Both partial phases have different properties with respect to their usability in solid electrolyte ceramics. One of these partial phases does not contain the additions of metal ions with a valence not exceeding 2 to a sufficient degree to guarantee a structure-related charge equalization of the mobile cations present without the assistance of lattice defects such as aluminum voids and/or interstitial oxygen ions. This is, so to speak, a partially stabilized phase. The second partial phase contains just as many additives of metal ions with a valence not exceeding 2, so that no further lattice defects other than the incorporation of these additives instead of aluminum ions are necessary for the structure-related charge equalization of the mobile cations present. This is therefore a fully stabilized phase.